(i) Field of the Invention
The present invention relates to an image output device for forming an image pattern in a dot matrix state, for example, a page printer such as a laser printer or a LED printer, or a CRT display, and an image control method for smoothing an image or increasing an image resolution by the use of the output device.
(ii) Description of the Prior Art
Heretofore, a laser printer has been known in which image patterns are formed in a dot matrix state on a photosensitive drum by repeatedly photo-scanning, in a main scanning direction and along buses on the drum, laser beams which have been modulated on the basis of video data serially output from an image controller.
Furthermore, a LED printer has also been known in which an arrayed LED head having LED elements linearly arranged in a main scanning direction is disposed along the bus of a photosensitive drum so as to confront the photosenstive drum, and the LED elements are controlled to drive (burn) one line at one time or every one block of the line on the basis of video data and the photosensitive drum is relatively moved in a secondary scanning direction, thereby forming an image pattern in a dot matrix state on the photosensitive drum.
In these kinds of printers, a processing for smoothing the image pattern or a processing for increasing an image resolution is necessary in order to obtain a sharper image, because the printers take a system in which an optional letter or graphic form is formed by suitably depicting a dot image pattern in an n.times.m dot matrix region on the basis of video data.
In the first place, the conventional technique of the processing for smoothing the image pattern will be described.
For example, in the case of forming a slanted line or a curved line such as "V" or "O", adjacent dots in the line are inconveniently formed in the state of a step. Additionally, the crossing portion of a letter such as "X" is made bold, because a plurality of dots are closely arranged, and in consequence, a printing quality deteriorates unavoidably.
In order to overcome such a drawback, a shift, i.e., a deviation of an attentional dot from the slant of an original line pattern is regarded as 1/2 of a distance between the attentional dot and a dot adjacent thereto, and presence or absence of the deviation of the attentional pixel is confirmed by ORing/ANDing the attentional dot or a blank (the dot which may be the blank will be generically called "pixel") with an adjacent pixel.
The above-mentioned dot means the pixel which will be printed by the adhesion of a toner on one corresponding point in an n.times.m dot matrix region, and the above-mentioned blank means the pixel showing a background point on which any toner will not adhere in the matrix region.
As described above, the deviation of the attentional dot which forms the step is at most 1/2 of a distance between the two dots (in the case that the slant of a slanted line which is the image pattern is 45.degree.). However, as the gradient of the slanted line is near to a vertical (horizontal) direction, a gap between the above-mentioned deviation and an original image increases. In other words, when the gradient of the slanted line is near to the vertical or the horizon, it is very difficult to smooth the step of the slanted line even by ORing/ANDing the adjacent pixels.
The step of the slanted line always contains the adjacent dot and blank in the main scanning direction and/or the secondary scanning direction. Thus, as described in Japanese Patent Application Laid-open No. 251761/1985, a technique has been suggested in which the blank adjacent to the attentional dot in the main scanning direction is replaced with a dot having a small energy density (which will be the dot having a small diameter on a photosensitive drum) to smooth the step portion. In this technique, however, the dot having the small diameter is merely added to the blank region adjacent to the attentional dot. Therefore, on a fine slanted or curved line, the dot-added portion becomes bold, which does not always lead to the improvement of the image quality. In addition, since this technique intends to add the dot to the blank in the main scanning direction, a smoothing effect is low, as the gradient of the slanted or curved line decreases.
In order to overcome the above-mentioned drawback, U.S. Pat. No. 4,847,641 (hereinafter referred to as "U.S. Pat. No. 641") has suggested a technique in which the dot having a small diameter is added to the blank adjacent to the attentional dot and the attentional dot itself is also narrowed (flattened) and apparently moved to one side, and the addition of the small-diameter dot and the flattening of the attentional dot are achieved not only in the main scanning (horizontal) direction but also in the secondary scanning (vertical) direction, in other words, in the two directions of right/left and top/bottom.
Next, the constitution of this conventional technique will be briefly described in reference to FIG. 9. In the first place, there are prepared four compensating subcells 51a, 51b, 51c and 51d which are narrowed in the main scanning direction and four compensating subcells 52a, 52b, 52c and 52d which are narrowed in the secondary scanning direction. Next, bit data (binary data arranged in a matrix state in a secondary scanning direction, and generally, 1 and 0 correspond to the dot and the blank, respectively) corresponding to video data are successively serially stored in an FIFO buffer 53 capable of storing the data every plural lines, and a plurality of upper, lower, right and left bit data adjacent to the attentional pixel are extracted through a sample window 54 and then forwarded to a matching network 55. In this matching network 55, the bit map data present in the sample window 54 are compared to many templates 56 stored in the matching network 55. If the bit data accord with the template 56, the compensating subcell 51a, 52a . . . for flattening the attentional dot and the compensating subcell 51a, 52a . . . for adding the flattened small dot to the upper, lower, right or left blank adjacent to the attentional dot are selected from a subcell generator 57. On the contrary, if the bit data do not accord with any templates 56, the standard pixel is selected as it is. Afterward, the thus selected video data are serially output to a print engine driver 58 to carry out printing.
However, this conventional technique also has some drawbacks. That is, in the practice of the technique, the two selecting operations must be carried out to select the subcells of the attentional pixel and the adjacent other pixels. Additionally, the four subcells are required for each of the horizontal direction and the vertical direction, and so the number of the templates necessary to compare the bit map data present in the sample window 54 with the templates is (2.sup.4).times.(2.sup.4)=256 in both the directions. In order to smooth the image patterns, application templates are further needful, and the total number of the templates is too large, which requires large-capacity memories for the large number of the templates and which unavoidably complicates and consequently enlarges a circuit constitution for the comparing operation.
Next, some problems associated with the increase in an image resolution will be described.
As described above, among page printers such as laser printers and LED printers, the printers having an image constitution of a low dot pitch density, for example, 300 dpi are often employed because of low costs, compaction and the like. In recent years, however, for the purpose of improving image quality and resolution, printers in which the dot pitch density is heightened to, for example, 600 dpi have been suggested.
However, in order to obtain the pixel image of 600 dpi, bit map data in an image processing circuit must be also developed with the similarly high density in data memory, which leads to not only the increase in memory capacity, the complication of a hardware constitution and the increase of the cost but also the retard of a processing rate.
In general, fonts of English and Japanese, already existent word processing softs and other application softs have been made on the basis of 300 dpi, and therefore, in the case that the 600 dpi pixel image is employed, they cannot be used as they are, with the result that the employment of the 600 dpi pixel image is devoid of prevalence.
Accordingly, it has been attempted that the bit map data developed in the state of 300 dpi are heightened to a high resolution level of 600 dpi by using a data conversion circuit, and then fed to the print engine driver.
For example, according to Japanese Patent Application Laid-open No. 59362/1990, there are used 3 line memories having a memory capacity corresponding to 600 dpi, a frequency multiplication circuit for doubling a video clock frequency of 300 dpi and a discriminant circuit for doing comparison/discrimination by logical sum, and the writing of bit map data in one line memory is carried out in parallel with the reading of the data from other two line memories by the use of a clock converted into 600 dpi in the frequency multiplication circuit. The thus read two data are ORed/ANDed in the discriminant circuit to output image data, and the image data of 600 dpi for each scanning line are successively output to a print engine driver of a laser printer by cyclically moving the above-mentioned line memory every one corresponding scanning line.
However, the foundation of the above-mentioned conventional technique resides in that the bit map data developed at 300 dpi in main and secondary scanning directions are enlarged simply twice every one scanning line, and the high-density bit map data of 600 dpi are then obtained by ORing/ANDing the attentional pixel with an adjacent reference bit. In other words, this technique is substantially equal to that the clock is halved and the original image data are enlarged twice by ORing/ANDing. This conventional technique can easily achieve the resolution enhancement of a slanted line at about 45.degree., but it cannot smoothly achieve that of the slanted line close to a horizontal line or a vertical line.
In order to overcome the above-mentioned drawback, a technique has also been suggested in which the original dot information of upper and lower 5 lines.times.7 bits adjacent to the attentional bit is stored, and the dot information is ORed/ANDed by a logical circuit to heighten resolution in consideration of a location error (Japanese Patent Application Laid-open No. 60764/1990).
However, it makes the circuit constitution of the hardware extremely complex to OR/AND the two-dimensional 5.times.7 dot information which is as much as 35 bits.
Furthermore, in the conventional technique, the enhancement of the resolution is limited to a two-fold level, and it is impossible to achieve a higher level of the resolution.